Addressing the increasing worldwide demand for energy requires strategies for more efficient energy use as well as sustainable approaches to energy generation and conservation. Lighting is a major contributor to electricity consumption, currently accounting for 19% of global use and 34% for the U.S. The U.S. lighting market is currently divided among various lamp types: 63% incandescent, 35% fluorescent, 2% halogen. Among these, the incandescent light bulb is only 5% efficient (15 lm/W) while the fluorescent lamp has 15-25% efficiency (50-80 lm/W). The appreciable energy savings that come with converting from incandescent to fluorescent lamps (including compact fluorescent lamps or CFLs) and solid-state lighting has motivated many national governments to pass regulations that phase out the use of incandescent lights.
CFLs are a candidate for replacement of incandescent light bulbs, but many find their spectrum to be less pleasing than traditional incandescent sources. Compact fluorescent lamps also pose environmental and health risks, e.g., because they contain very toxic mercury vapor. Also, traditional cathode ray tubes and similar devices use electrons to stimulate conventional phosphors, typically transition metal or rare earth compounds. The conventional phosphors limit efficiency because the phosphors must dissipate charge by emitting a photon before accepting additional energy.
Solid state light devices such as light emitting diodes (LEDs) made of semiconductors, and particularly planar devices, are of great interest and have been studied broadly. Such LEDs are also in the process of entering the market. LEDs have the potential to revolutionize the lighting industry with higher efficiency, better quality and lower maintenance, and could to a reduction by half of energy consumed by general illumination.
However, traditional incandescent light sources are favored by many people because they provide a broad spectrum of incoherent light that produces a pleasing white light. A given LED, on the other hand, emits light of a specific color determined by the bandgap of the semiconductor material constituting the LED.
One approach for producing white light is to use multiple LEDs of different colors—e.g., red (R), green (G), and blue (B). Another approach is to use phosphors to transform blue or near-UV light from an LED, e.g., a GaN-based LED, to “pump” a phosphor or mixture of phosphors. The multiple LED approach leads to narrow spectral lines and is limited in practice by the low efficiency of green LEDs. On the other hand, conventional blue LEDs coated with yellow phosphors give a cold white light and are not color tunable.
Further, although conventional LEDs offers intrinsic advantages of high energy conversion efficiency, they have problems in that the thin film based solid stable LEDs suffer from low light extraction efficiency due to total internal reflection (TIR). Internal waveguiding and self-absorption cause heating and reduce the light emission efficiency and device lifetime. Typically, a heat sink is needed for light bulbs with power higher than 60 W, and the overall energy conversion efficiency is about 30-35%. Cost is also an issue for the manufacturing of the LED devices.
Extensive research efforts have been made towards improving light extraction efficiency. Among the different approaches tested, fabricating vertical arrays of one-dimensional semiconductor devices, such as nanowire (NW) and/or pillar LEDs, has been of great interest. NWs, especially, are considered to have an advantage of high light extraction efficiency due to the waveguide effect and photonic crystal effect. Furthermore, nanowires have been demonstrated to serve as a very good lasing cavity, and both optical and electrical driven NW lasers have been reported.
However, NW-based LEDs generally require the formation of window electrodes on their top surfaces for current injection. These top electrodes in NW-based LEDs may block the emitting light and cause ineffective carrier injection into the NWs. The established approach to fabricate such electrodes involves filling the gap between the NWs or pillars with other materials, such as polymers or spin-on glass, and coating with conductive layers.
Moreover, for most of the wide bandgap semiconductors, hole injection into p-type materials has proved problematic. This is mainly due to the low electrical conductivity in p-type materials, which can therefore inhibit the effective hole injection, the electron-hole recombination, and thus the performance of NW-based LEDs.